This invention relates to testing methods and apparatuses for digital signal processing systems, particularly to methods and apparatuses for testing a digital signal processing system for the occurrence of errors. The term "digital signal processing system" as used herein is intended to include systems for digital signal processing, transmission, recording and other digital systems that perform similar functions.
In testing digital signal processing systems it is ordinarily desirable to determine whether or not an error in a digital bit transmitted by the system has occurred over a selected period of time, that is the "error rate."
The conventional method for testing digital signal processing systems is to apply a pseudo random data pattern signal to the input of the system and examine the pattern of the output signal in comparison to the input signal to determine whether they match. This allows very sensitive testing, since an error in any bit can be identified. Pseudo random data patterns permit examination of all possible codes of the system under test, and will simulate the dynamic characteristics of a signal of the type normally applied to the system.
While the pseudo random data pattern is ideal for testing many digital signal transmission systems, it will not work with systems that introduce any signal processing, that is, where the transfer function of the system is unknown or variable. This is because it is difficult, if not impossible, to predict with sufficient accuracy what the effect of a particular adjustment of the system will be on the transfer function. For example, some digital audio mixing consoles will not pass digital signals without some modification, no matter how carefully the controls are adjusted. This may be due to many reasons, such as inadequate resolution in fader settings, a high pass filter in the data path, or the presence of equalizers. On the other hand, a digital telephone transmission system typically has a predictable, fixed transfer function when it is properly operating, so it can be tested using pseudo random data.
Some manufacturers of audio digital signal transmission systems provide their equipment with test routines that fix the adjustments in a known way so that the input can be compared to the output. However, such routines do not permit testing under normal operating conditions, and they require the addition of special purpose hardware or software to the system just for testing, which increases the cost of the system.
Accordingly, it would be desirable to have, and there is a need for, a method and apparatus that will determine in a digital signal processing system whose transfer function is unpredictable, whether or not a processing error has occurred, when it has occurred, and what the rate of processing errors is.